Secondary battery and electric device

Abstract

The invention provides a secondary battery and an electric device, and belongs to the technical field of batteries. According to the secondary battery provided by the invention, the negative pole piece of the flat part of the roll core is provided with different areas from inside to outside, and the areas and unit area capacities of the different areas meet a certain proportional relation; therefore, the lithium embedding amount of different areas of the negative pole piece of the flat part of the roll core tends to be balanced in the charge-discharge cycle process of the obtained secondary battery, so that uniform lithium embedding of the negative pole piece when the secondary battery is fully charged is realized, the cycle diving time of the secondary battery is delayed, and the cycle performance and the safety performance of the secondary battery are improved.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a secondary battery and an electrical device. Background Art

[0002] Due to the uneven distribution of the electric field when lithium ions are charged and discharged, the flat part of the core is unevenly stressed during hot pressing, resulting in differences such as different bonding forces and gaps with the diaphragm from the center to the edge of the negative electrode sheet on the flat part of the core, which makes the flat part preferentially embedded with lithium during the cycle. After the lithium is embedded in the center of the negative electrode sheet on the flat part, some of the surrounding lithium will be embedded on the surface of the negative electrode sheet, forming a thin lithium layer that is invisible to the naked eye. Most of this thin lithium deposition layer is reversible. However, as the cycle progresses, this part of irreversible lithium gradually accumulates in the central area of ​​the negative electrode sheet on the flat part of the core, causing lithium deposition in the center of the negative electrode sheet on the flat part. As lithium deposition occurs, the lithium deposition potential of the core becomes lower and lower, causing a diving phenomenon, which terminates the life of the core and is prone to safety hazards. Especially under low temperature or fast charging conditions, the difference in lithium insertion rate between the middle area and the surrounding area of ​​the negative electrode sheet on the flat part increases, so the diving phenomenon caused by lithium deposition in the center of the negative electrode sheet on the flat part is particularly obvious. Summary of the invention

[0003] The purpose of the present application is to solve the problem of dead lithium accumulation and regional lithium plating caused by preferential lithium embedding in the center of the negative electrode plate on the flat part of the existing secondary battery during fast charging cycle, thereby significantly reducing the cycle performance and safety performance of the secondary battery, and to provide a secondary battery and electrical device that can effectively delay the cycle diving time of the secondary battery, increase the service life of the secondary battery and the safety.

[0004] To achieve the above-mentioned object, in a first aspect of the present application, a secondary battery is provided, comprising a winding core, the winding core having a flat portion and a curved portion, the winding core comprising a positive electrode sheet, a separator and a negative electrode sheet arranged in sequence, the negative electrode sheet comprising a negative electrode current collector and a negative electrode active material layer distributed on at least one surface of the negative electrode current collector;

[0005] The negative electrode sheet of the flat portion is provided with a first area and a second area in sequence from the inside to the outside, the second area surrounds the edge of the first area, and the center of the first area overlaps with the center of the flat portion;

[0006] The area of ​​the first region is S1, and the area of ​​the second region is S2, satisfying: 0.1≤S2 / S1≤2.2; the capacity per unit area of ​​the first region is a, and the capacity per unit area of ​​the second region is b, satisfying: 1<a / b<1.3.

[0007] As an embodiment of the present application, the secondary battery satisfies: 0.5≤S2 / S1≤1.2, 1.1≤a / b<1.2.

[0008] As an embodiment of the present application, a is 2.2 to 4.5 mAh / cm 2 , b is 1.8~4.1mAh / cm 2 .

[0009] As an implementation scheme of the present application, the second region includes a first sub-region and a second sub-region, the second sub-region surrounds the edge of the first sub-region, the first sub-region surrounds the edge of the first region, the unit area capacity of the first sub-region is b1, and the unit area capacity of the second sub-region is b2, satisfying: b2<b1,1<b1 / b2≤1.1.

[0010] As an embodiment of the present application, b1 is 2.0 to 4.1 mAh / cm 2 , b2 is 1.8~3.8mAh / cm 2 .

[0011] As an implementation scheme of the present application, the area of ​​the first sub-region is S21, and the area of ​​the second sub-region is S22, satisfying: 0.8≤S22 / S21≤1.2.

[0012] As an implementation scheme of the present application, the surface density of the first region is CW1, the surface density of the first sub-region is CW21, and the surface density of the second sub-region is CW22, satisfying: CW22<CW21<CW1.

[0013] As an embodiment of the present application, CW1 is 6.5 to 13 mg / cm 2 , CW21 is 5.9~11.8mg / cm 2 , CW22 is 5.4~10.8mg / cm 2 .

[0014] As an embodiment of the present application, the negative electrode sheet of the bent portion has a capacity per unit area of ​​c, and c is 2.0 to 4.1 mAh / cm 2 .

[0015] As an embodiment of the present application, the negative electrode active material layer contains artificial graphite and a silicon-based material, the content of the silicon-based material in the first region is Si1, the content of the silicon-based material in the first sub-region is Si21, and the content of the silicon-based material in the second sub-region is Si22, satisfying: Si22<Si21<Si1.

[0016] In a second aspect of the present application, an electrical device is provided, comprising the secondary battery described above.

[0017] Compared with the prior art, the beneficial effects of this application are:

[0018] A secondary battery provided by the present application is configured with different regions of the negative electrode sheet on the flat part of the winding core from the inside to the outside, and the area and unit area capacity of the different regions satisfy a certain proportional relationship, so that the lithium insertion amounts of different regions of the negative electrode sheet on the flat part of the winding core of the obtained secondary battery tend to be balanced during the charge and discharge cycle, thereby achieving uniform lithium insertion of the negative electrode sheet when the secondary battery is fully charged, thereby delaying the cycle diving time of the secondary battery and improving the cycle performance and safety performance of the secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 Schematic diagram of the winding core structure;

[0020] Figure 2 It is a schematic diagram of the negative electrode plate partitioning of the flat portion, in which A1 is the first area of ​​the negative electrode plate, A2 is the second area of ​​the negative electrode plate, A21 is the first sub-area of ​​the second area, and A22 is the second sub-area of ​​the second area. DETAILED DESCRIPTION

[0021] In order to make the purpose, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application are described clearly and completely below. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present application.

[0022] In the present application, the technical features described in an open manner include closed technical solutions composed of the listed features, and also include open technical solutions containing the listed features.

[0023] In this application, when it comes to numerical ranges, unless otherwise specified, the above numerical ranges are deemed to be continuous and include the minimum and maximum values ​​of the range, as well as each value between such minimum and maximum values. Further, when a range refers to an integer, each integer between the minimum and maximum values ​​of the range is included. In addition, when multiple ranges are provided to describe features or characteristics, the ranges can be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges included therein.

[0024] The reagents or instruments used in this application without indicating the manufacturer are all conventional products that can be obtained through commercial purchase.

[0025] Reference Figure 1 and Figure 2As shown, the present application proposes a secondary battery, comprising a winding core, the winding core having a flat portion and a curved portion, the winding core comprising a positive electrode sheet, a separator and a negative electrode sheet arranged in sequence, the negative electrode sheet comprising a negative electrode current collector and a negative electrode active material layer distributed on at least one surface of the negative electrode current collector;

[0026] The negative electrode sheet of the flat portion is provided with a first area and a second area in sequence from the inside to the outside, the second area surrounds the edge of the first area, and the center of the first area overlaps with the center of the flat portion;

[0027] The area of ​​the first region is S1, and the area of ​​the second region is S2, satisfying: 0.1≤S2 / S1≤2.2; the capacity per unit area of ​​the first region is a, and the capacity per unit area of ​​the second region is b, satisfying: 1<a / b<1.3.

[0028] The present application sets a first area and a second area in sequence from the inside to the outside of the negative electrode sheet on the flat part of the winding core, and the center of the first area overlaps with the center of the flat part, and the second area surrounds the edge of the first area. At the same time, the present application limits the ratio of the area S1 of the first area to the area S2 of the second area to satisfy 0.1≤S2 / S1≤2.2 and the ratio of the unit area capacity a of the first area to the unit area capacity b of the second area to satisfy 1<a / b<1.3, so that the lithium insertion amount of different areas of the negative electrode sheet on the flat part of the winding core of the secondary battery can tend to be balanced during the charge and discharge cycle, thereby achieving uniform lithium insertion of the negative electrode sheet when the secondary battery is fully charged, which can effectively suppress the local lithium insertion phenomenon of the negative electrode sheet, thereby improving the cycle performance and safety performance of the secondary battery.

[0029] The present application has no particular limitation on the shapes of the first region and the second region, for example, they may be square, rectangular, etc.

[0030] It should be noted that the areas of the first and second regions are measured using a ruler and then calculated. Calibration of the first and second regions: disassemble the battery cell, take out the negative electrode sheet, soak and clean it with DMC or EMC, and then dry it. Cut a negative electrode sheet (including the flat part and the curved part), test the diaphragm resistance of the entire electrode sheet at intervals of 1 to 2 mm, and draw a diaphragm resistance diagram of the electrode sheet surface. Draw different area shapes according to the difference in diaphragm resistance to distinguish S1 and S2; S1 and S2 are further confirmed by surface capacity.

[0031] Exemplarily, the ratio between the area S1 of the first region and the area S2 of the second region satisfies 0.1≤S2 / S1≤2.2, that is, S2 / S1 can be any point value or any two point range values ​​between 0.1 and 2.2, for example, it can be 0.1, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, 2.2, etc.

[0032] It should be noted that the capacity per unit area of ​​the first region and the second region can be obtained by assembling a half-cell and performing a button test: take the negative electrode sheets of the first region and the second region respectively, wipe off the active layer on one side with water, cut the single-sided negative electrode sheet into small discs, weigh them, and then assemble them into button batteries with lithium sheets, diaphragms, nickel meshes, and electrolytes. After charging and discharging three times with a current of 0.01mA and 0.005V to 2V, the corresponding capacity C mAh / g is obtained.

[0033] Exemplarily, the ratio of the capacity per unit area a of the first region to the capacity per unit area b of the second region satisfies 1<a / b<1.3, that is, a / b can be any point value or any two point range values ​​between greater than 1 and less than 1.3, for example, it can be 1.01, 1.02, 1.04, 1.06, 1.08, 1.1, 1.12, 1.14, 1.16, 1.18, 1.2, 1.22, 1.24, 1.26, 1.28, 1.29, etc.

[0034] In one embodiment, the secondary battery satisfies: 0.5≤S2 / S1≤1.2, 1.1≤a / b<1.2. For example, S2 / S1 can be 0.5, 0.7, 0.9, 1.1, 1.2, etc.; a / b can be 1.1, 1.13, 1.15, 1.17, 1.19, etc.

[0035] The present application has found that, firstly, during the charge and discharge process of the secondary battery, the potential of the current collector is the same, a flat capacitor effect is formed between the positive and negative electrodes, and the magnetic field strength in the middle area of ​​the flat negative electrode plate is stronger than that in the surrounding area, resulting in a faster rate of ion migration from the positive electrode to the negative electrode; secondly, during the charge and discharge process of the secondary battery, the heat generated by the internal resistance will also make the temperature in the middle area of ​​the flat negative electrode plate higher than that in the edge area, and the increase in temperature will further accelerate the ion migration and the electrochemical reaction rate; the above two aspects lead to a significantly higher lithium insertion rate in the middle area of ​​the flat negative electrode plate than in the surrounding area, and the cumulative effect increases with the increase in the number of cycles, and after increasing to a certain extent, lithium is preferentially deposited in the middle area of ​​the flat negative electrode plate. The present application further optimizes the secondary battery to meet the following conditions: 0.5≤S2 / S1≤1.2, 1.1≤a / b<1.2, so as to better reduce the difference in the amount of lithium inserted in the middle area and the surrounding area of ​​the flat negative electrode plate, and realize uniform lithium insertion of the flat negative electrode plate, thereby achieving better cycle performance and safety performance.

[0036] In one embodiment, a is 2.2 to 4.5 mAh / cm 2 , b is 1.8~4.1mAh / cm 2 .

[0037] For example, the a may be 2.2 to 4.5 mAh / cm 2 Any point value or any range value between two points, such as 2.2mAh / cm 2 、2.4mAh / cm 2 、2.6mAh / cm 2 、2.8mAh / cm 2 、3.0mAh / cm 2 、3.2mAh / cm 2 、3.4mAh / cm 2 、3.6mAh / cm 2 、3.8mAh / cm 2 、4.0mAh / cm 2 、4.2mAh / cm 2 、4.4mAh / cm 2 、4.5mAh / cm 2 etc.; b can be 1.8~4.1mAh / cm 2 Any point value or any range value between two points, such as 1.8mAh / cm 2 、2.0mAh / cm 2 、2.2mAh / cm 2 、2.4mAh / cm 2 、2.6mAh / cm 2 、2.8mAh / cm 2 、3.0mAh / cm 2 、3.2mAh / cm 2 、3.4mAh / cm 2 、3.6mAh / cm 2 、3.8mAh / cm 2 、4.0mAh / cm 2 、4.1mAh / cm 2 wait.

[0038] The present application has found that the capacity per unit area reflects the amount of charge that can be stored per unit area under a given potential difference. The capacity per unit area within an appropriate range can not only achieve a good energy density of the secondary battery, but also achieve excellent stability and low internal resistance of the secondary battery. When the present application controls the capacity per unit area a of the first region and the capacity per unit area b of the second region to satisfy 1<a / b<1.3, a is further selected to be 2.2~4.5mAh / cm 2 , b is 1.8~4.1mAh / cm 2 , which can make the obtained secondary battery have not only good cycle performance and safety performance, but also excellent energy density.

[0039] In one embodiment, the second area includes a first sub-area and a second sub-area, the second sub-area surrounds the edge of the first sub-area, the first sub-area surrounds the edge of the first area, the unit area capacity of the first sub-area is b1, and the unit area capacity of the second sub-area is b2, satisfying: b2<b1,1<b1 / b2≤1.1.

[0040] Exemplarily, the ratio b1 / b2 of the unit area capacity b1 of the first sub-area to the unit area capacity b2 of the second sub-area can be any point value or any two point range values ​​greater than 1 and less than or equal to 1.1, such as 1.01, 1.02, 1.03, 1.04, 1.05, 1.06, 1.07, 1.08, 1.09, 1.1, etc.

[0041] The present application has found that further setting the first sub-region and the second sub-region in the second region, and limiting the unit area capacity b1 of the first sub-region and the unit area capacity b2 of the second sub-region to satisfy b2<b1,1<b1 / b2≤1.1, can more effectively reduce the difference in the amount of lithium embedded in the middle region and the surrounding region of the flat negative electrode sheet during the charge and discharge process, thereby helping to achieve good cycle performance and safety performance of the secondary battery.

[0042] In one embodiment, b1 is 2.0-4.1 mAh / cm 2 , b2 is 1.8~3.8mAh / cm 2 .

[0043] For example, b1 may be 2.0 to 4.1 mAh / cm 2 Any point value or any range value between two points, such as 2.0mAh / cm 2 、2.2mAh / cm 2 、2.4mAh / cm 2 、2.6mAh / cm 2 、2.8mAh / cm 2 、3.0mAh / cm 2 、3.2mAh / cm 2 、3.4mAh / cm 2 、3.6mAh / cm 2 、3.8mAh / cm 2 、4.0mAh / cm 2 、4.1mAh / cm 2 etc.; b2 can be 1.8~3.8mAh / cm 2 Any point value or any range value between two points, such as 1.8mAh / cm 2 、2.0mAh / cm 2、2.2mAh / cm 2 、2.4mAh / cm 2 、2.6mAh / cm 2 、2.8mAh / cm 2 、3.0mAh / cm 2 、3.2mAh / cm 2 、3.4mAh / cm 2 、3.6mAh / cm 2 、3.8mAh / cm 2 wait.

[0044] The present application has found that when the capacity per unit area of ​​the first sub-region and the second sub-region in the second region is further selected to be within the above ranges, the secondary battery can have a good energy density while reducing the difference in the amount of lithium embedded in the flat negative electrode sheet.

[0045] In one embodiment, the area of ​​the first sub-region is S21, and the area of ​​the second sub-region is S22, satisfying: 0.8≤S22 / S21≤1.2.

[0046] Exemplarily, the ratio S22 / S21 of the area S21 of the first sub-region to the area S22 of the second sub-region may be any point value or any two point range values ​​between 0.8 and 1.2, such as 0.8, 0.9, 1.0, 1.1, 1.2, etc.

[0047] The research in this application found that when the ratio of the area of ​​the first sub-region and the second sub-region in the second region is further selected to be within the above range, the flat negative electrode plate can be better partitioned with a good regional gradient, thereby achieving excellent comprehensive effects of the secondary battery on the basis of reducing the difficulty of the preparation process.

[0048] In one embodiment, the surface density of the first region is CW1, the surface density of the first sub-region is CW21, and the surface density of the second sub-region is CW22, satisfying: CW22<CW21<CW1.

[0049] It should be noted that the surface density of the first region, the first sub-region and the second sub-region is obtained by testing the small disc weighing method. The specific testing process is: take the negative electrode plate and use deionized water to wipe off the active layer. The mass of the small disc cut from the foil is m0, that is, the mass of the foil, and the area of ​​the small disc is A1. Then, the pole pieces of the first region, the first sub-region and the second sub-region are cut into small discs (including the active layer and the foil), and the weights are recorded as m1, m21 and m22 respectively. Then CW1=(m1-m0) / A1, CW21=(m21-m0) / A1, CW22=(m22-m0) / A1.

[0050] The research of this application found that the surface density of the flat negative electrode plate is related to the specific capacity to a certain extent. The higher the surface density, the stronger the corresponding ion storage and release capacity. This application can effectively avoid the problem of excessive lithium insertion speed in the middle area of ​​the flat negative electrode plate due to the magnetic field or internal resistance during the charging and discharging of the secondary battery by selecting the surface density CW1 of the first area, the surface density CW21 of the first sub-area and the surface density CW22 of the second sub-area to satisfy W22<W21<W1, thereby achieving uniform lithium insertion of the entire flat negative electrode plate, reducing the risk of lithium plating, and improving the cycle performance and safety performance of the secondary battery.

[0051] In one embodiment, CW1 is 6.5-13 mg / cm 2 , CW21 is 5.9~11.8mg / cm 2 , CW22 is 5.4~10.8mg / cm 2 .

[0052] For example, CW1 may be 6.5 to 13 mg / cm 2 Any point value or any range value between two points, such as 6.5mg / cm 2 、7.0mg / cm 2 、7.5mg / cm 2 、8.0mg / cm 2 、8.5mg / cm 2 , 9.0mg / cm 2 、9.5mg / cm 2 、10.0mg / cm 2 、10.5mg / cm 2 、11.0mg / cm 2 、11.5mg / cm 2 、12.0mg / cm 2 , 12.5mg / cm 2 、13.0mg / cm 2 etc.; CW21 can be 5.9~11.8mg / cm 2 Any point value or any range value between two points can be obtained, for example, 5.9 mg / cm 2 、6.5mg / cm 2 、7.0mg / cm 2 、7.5mg / cm 2 、8.0mg / cm 2 、8.5mg / cm 2 , 9.0mg / cm 2 、9.5mg / cm 2 、10.0mg / cm 2、10.5mg / cm 2 、11.0mg / cm 2 、11.5mg / cm 2 、11.8mg / cm 2 etc.; CW22 can be 5.4~10.8mg / cm 2 Any point value or any range value between two points, such as 5.4mg / cm 2 , 5.9mg / cm 2 、6.5mg / cm 2 、7.0mg / cm 2 、7.5mg / cm 2 、8.0mg / cm 2 、8.5mg / cm 2 , 9.0mg / cm 2 、9.5mg / cm 2 、10.0mg / cm 2 、10.8mg / cm 2 wait.

[0053] The research in this application found that the surface density of different regions of the flat negative electrode sheet is not only related to the internal resistance and capacity of the secondary battery, but also to the cycle performance of the secondary battery. When the surface density of different regions of the flat negative electrode sheet is further selected within the above range, the comprehensive effect of the secondary battery obtained is better.

[0054] In one embodiment, the capacity per unit area of ​​the negative electrode plate of the bent portion is c, and c is 2.0-4.1 mAh / cm 2 .

[0055] For example, the negative electrode sheet of the bent portion may have a unit area capacity of 2.0 to 4.1 mAh / cm 2 Any point value or any range value between two points, such as 2.0mAh / cm 2 、2.2mAh / cm 2 、2.5mAh / cm 2 、2.8mAh / cm 2 、3.0mAh / cm 2 、3.2mAh / cm 2 、3.5mAh / cm 2 、3.8mAh / cm 2 、4.0mAh / cm 2 、4.1mAh / cm 2 wait.

[0056] The present application found that the unit area capacity of the negative electrode sheet of the bent portion will affect the energy density of the secondary battery, and further affect the cycle performance and safety performance of the secondary battery. When the unit area capacity of the negative electrode sheet of the bent portion of the secondary battery is further selected to be 2.0-4.1 mAh / cm 2 When the secondary battery is used, the overall performance is better.

[0057] In one embodiment, the negative electrode active material layer comprises artificial graphite and a silicon-based material, the content of the silicon-based material in the first region is Si1, the content of the silicon-based material in the first sub-region is Si21, and the content of the silicon-based material in the second sub-region is Si22, satisfying: Si22<Si21<Si1.

[0058] It should be noted that the content of silicon-based materials in the first region, the first sub-region and the second sub-region is obtained by ICP elemental analysis, specifically: the powder of the active layer scraped off the negative electrode sheets of the first region, the first sub-region and the second sub-region is measured by ICP (inductively coupled plasma atomic emission spectrometry) for Si element content.

[0059] The research in this application found that the content of silicon-based materials in the first region, the first sub-region and the second sub-region will affect the storage and release rates of ions in different regions, thereby affecting the lithium insertion rate and amount of lithium insertion in different regions of the flat negative electrode sheet. When the content of silicon-based materials in the first region, the first sub-region and the second sub-region is further selected to satisfy Si22<Si21<Si1, the difference in the amount of lithium insertion between the middle region and the surrounding region of the flat negative electrode sheet can be effectively reduced, thereby avoiding the occurrence of severe lithium plating and achieving excellent cycle performance and safety performance of the secondary battery.

[0060] In one embodiment, the silicon-based material includes at least one of single crystal silicon, polycrystalline silicon, amorphous silicon, silicon carbon, and silicon oxygen.

[0061] In one embodiment, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector; the positive electrode active material layer includes a positive electrode active material. The present application has no limitation on the positive electrode active material, and any known positive electrode active material can be used, such as the positive electrode active material includes at least one of a ternary positive electrode material, a lithium iron phosphate positive electrode material, or a lithium iron manganese phosphate positive electrode material.

[0062] The present application has no limitation on the diaphragm, and any known diaphragm may be used.

[0063] An embodiment of the present application provides an electrical device, comprising the secondary battery described above.

[0064] Exemplarily, the above-mentioned electrical devices may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but are not limited thereto.

[0065] Example 1

[0066] The present invention provides a secondary battery, wherein a method for preparing the secondary battery comprises the following steps:

[0067] 1) Preparation of negative electrode sheet

[0068] After the negative electrode active material (artificial graphite and silicon dioxide in a mass ratio of 9:1), the conductive agent (conductive carbon black and carbon nanotubes in a mass ratio of 1:1), sodium carboxymethyl cellulose and styrene-butadiene rubber are uniformly mixed in a mass ratio of 96:1:1:2, deionized water is added and stirred to obtain a negative electrode slurry, and the negative electrode slurry is double-sidedly coated on the negative electrode current collector by an extrusion coating device and fully dried to obtain a negative electrode active material layer, and the obtained negative electrode current collector and the negative electrode active material layer are integrally rolled to obtain a negative electrode sheet. During the coating process, the coating area and coating weight parameters are controlled so that the flat part (the negative electrode sheet in the flat part has a first area and a second area with different unit area capacities, and the second area includes a first sub-area and a second sub-area) and the curved part of the negative electrode sheet after winding meet the characteristics shown in Table 1.

[0069] 2) Preparation of positive electrode

[0070] The positive electrode material (lithium iron phosphate), conductive carbon black and polyvinylidene fluoride are uniformly mixed in a mass ratio of 96:2:2, and then put into N-methylpyrrolidone and stirred to obtain a positive electrode slurry. The positive electrode slurry is double-sidedly coated on the positive electrode collector by extrusion coating equipment and fully dried to obtain a positive electrode active material layer. The positive electrode collector and the positive electrode active material layer are roll-pressed as a whole to obtain a positive electrode sheet.

[0071] 3) Preparation of electrolyte

[0072] Ethylene carbonate, ethyl methyl carbonate and diethyl carbonate were mixed in a volume ratio of 1:1:1, and then 1 mol / L LiPF6 was added and mixed evenly to obtain an electrolyte.

[0073] 4) Preparation of secondary batteries

[0074] The positive electrode sheet, the separator, and the negative electrode sheet are stacked and rolled in order. The separator is a polyethylene film, which is placed between the positive and negative electrode sheets to play a role of isolation. The winding core is obtained. The schematic diagram of the winding core structure is shown in the figure. Figure 1As shown; then the roll core is placed in the punched aluminum-plastic film for packaging, and the secondary battery is obtained after liquid injection and formation to a fixed capacity.

[0075] Embodiments 2 to 13

[0076] The coating parameters were controlled so that Examples 2 to 13 met the characteristics shown in Table 1.

[0077] Comparative Examples 1-2

[0078] The coating parameters are controlled so that comparative examples 1 to 2 meet the characteristics shown in Table 1 (the capacity per unit area of ​​the first and second regions of the negative electrode plate in comparative example 1 is the same, and the capacity per unit area of ​​the first region of the negative electrode plate in comparative example 2 is less than the capacity per unit area of ​​the second region).

[0079] Table 1

[0080]

[0081] In Table 1: the unit of a, b, a1, b1, c is mAh / cm 2 , the unit of CW1, CW21, CW22 is mg / cm 2 .

[0082] Effect example

[0083] The energy density, cycle performance and lithium precipitation degree of the secondary batteries prepared in the embodiments and comparative examples were investigated, including the following tests:

[0084] 1) Energy density: Weigh the mass of the secondary battery (Kg), then place it in a 25℃ environment for 1h, charge it to 4.25V at a constant current and constant voltage of 0.33C, and the cut-off current is 0.05C; discharge it at a constant current of 0.33C, and the cut-off voltage is 2.8V. This is a complete charge and discharge cycle test. Repeat the charge and discharge cycle 3 times, and obtain the discharge energy (Wh) from the third discharge test. Energy density = discharge energy / battery mass.

[0085] 2) Cycle performance: At 25°C, charge at a constant current of 0.33C to 4.25V, then charge at a constant voltage of 4.25V until the current is less than 0.05C. After standing for 5 minutes, discharge at a current of 0.33C to 2.8V to obtain the discharge capacity. Record the number of cycles corresponding to the time when the discharge capacity decays to 80% of the first discharge capacity.

[0086] 3) Lithium deposition degree: judged by the surface color and area size when the secondary battery is fully charged. Lithium deposition area calibration: fully charge the battery to 4.25V at a 5C rate, disassemble it in a dry room, separate the fully charged negative electrode, take photos of the front and back of the entire electrode, process the electrode in the photo according to the color to obtain the area of ​​various colors, calculate the ratio to the entire electrode area, and record the lithium deposition status of the electrode. The lithium deposition status is recorded according to the standards shown in Table 2.

[0087] Table 2

[0088] Lithium level Judgment criteria No lithium deposition Pure yellow (golden yellow), no white-gray substance visible to the naked eye Slight lithium deposition A small number of white or gray spots, less than 10% of the pole piece area Moderate lithium precipitation Partial white or gray spots, covering 20-40% of the pole piece area Severe lithium evolution Partial white or gray spots, the area is greater than 40% of the pole piece area

[0089] The test results are recorded in Table 3;

[0090] Table 3

[0091]

[0092]

[0093] As can be seen from Table 3, the present application sets different areas on the negative electrode sheet of the flat part of the secondary battery core, and the different areas set meet the area ratio and unit area capacity ratio within a specific range. The corresponding secondary battery has excellent cycle performance while maintaining a high capacity, and can effectively inhibit the occurrence of lithium deposition on the negative electrode sheet, thereby improving safety performance. Specifically, the number of cycles when the capacity of the secondary battery is attenuated to 80% is more than 1,800, and the lithium deposition level after high-rate cycling is no lithium deposition or slight lithium deposition.

[0094] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A secondary battery, comprising a winding core, the winding core having a flat portion and a curved portion, the winding core comprising a positive electrode sheet, a separator and a negative electrode sheet arranged in sequence, characterized in that: The negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer distributed on at least one surface of the negative electrode current collector; The negative electrode sheet of the flat portion is provided with a first area and a second area in sequence from the inside to the outside, the second area surrounds the edge of the first area, and the center of the first area overlaps with the center of the flat portion; The area of ​​the first region is S1, and the area of ​​the second region is S2, satisfying: 0.1≤S2 / S1≤2.2; the capacity per unit area of ​​the first region is a, and the capacity per unit area of ​​the second region is b, satisfying: 1<a / b<1.

3.

2. The secondary battery according to claim 1, characterized in that: The secondary battery satisfies: 0.5≤S2 / S1≤1.2, 1.1≤a / b<1.

2.

3. The secondary battery according to claim 1, characterized in that: a is 2.2~4.5mAh / cm 2 , b is 1.8~4.1mAh / cm 2 .

4. The secondary battery according to claim 1, characterized in that: The second area includes a first sub-area and a second sub-area, the second sub-area surrounds the edge of the first sub-area, the first sub-area surrounds the edge of the first area, the unit area capacity of the first sub-area is b1, and the unit area capacity of the second sub-area is b2, satisfying: b2<b1,1<b1 / b2≤1.

1.

5. The secondary battery according to claim 4, characterized in that: b1: 2.0~4.1mAh / cm 2 , b2 is 1.8~3.8mAh / cm 2 .

6. The secondary battery according to claim 4, characterized in that: The area of ​​the first sub-region is S21, and the area of ​​the second sub-region is S22, satisfying: 0.8≤S22 / S21≤1.

2.

7. The secondary battery according to claim 4, characterized in that: The surface density of the first region is CW1, the surface density of the first sub-region is CW21, and the surface density of the second sub-region is CW22, satisfying: CW22<CW21<CW1.

8. The secondary battery according to claim 7, characterized in that: CW1: 6.5~13mg / cm 2 , CW21 is 5.9~11.8mg / cm 2 , CW22 is 5.4~10.8mg / cm 2 .

9. The secondary battery according to claim 1, characterized in that: The negative electrode plate of the bent portion has a unit area capacity of c, which is 2.0 to 4.1 mAh / cm 2 .

10. The secondary battery according to claim 4, characterized in that: The negative electrode active material layer comprises artificial graphite and silicon-based material, the content of the silicon-based material in the first region is Si1, the content of the silicon-based material in the first sub-region is Si21, and the content of the silicon-based material in the second sub-region is Si22, satisfying: Si22<Si21<Si1.

11. An electrical device, characterized in that: A secondary battery comprising the secondary battery according to any one of claims 1 to 9.

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